1. Field of the Invention
The present invention relates to a stepping motor, and, more particularly, to a hybrid-type stepping motor used in, for example, an inkjet printer or a scanner.
2. Description of the Related Art
An example of a known conventional hybrid-type stepping motor is illustrated in FIG. 7.
The stepping motor 1 includes a three-phase six-pole stator 6; a pair of brackets 7a and 7b, one externally fitted to each end portion of the stator 6 in an axial direction thereof; and a rotor 9 inserted in the inner peripheral side of the stator 6 and rotatably supported by the pair of brackets 7a and 7b through bearings 8a and 8b.
The rotor 9 comprises a rotary shaft 10; a disc-shaped permanent magnet 11 secured to the rotary shaft 10; and gear-like rotor cores 12a and 12b, each of which is secured to a corresponding end of the permanent magnet 11 in an axial direction thereof.
The rotor core 12a is separated from the rotor core 12b by half a pitch in a peripheral direction thereof, and is positioned so that a toothed pole (not shown) of the rotor core 12b is disposed between adjacent toothed poles (not shown) of the rotor core 12a. In FIG. 7, reference numeral 15 denotes a bolt for securing the pair of brackets 7a and 7b to the stator 6.
It is well known that hybrid-type stepping motors have a high noise level and vibrate considerably. In reality, however, a technology which allows effective reduction of such noise and vibration has not yet been proposed.
The present inventors have analyzed the causes of noise and vibration in hybrid-type stepping motors. It was found that for a standard hybrid-type stepping motor with an air gap of 50 .mu.m, when the generated tangent force that turns into torque was 1, the radial attraction/repulsion force (the vibration force) between the stator 6 and the rotor 9 was over 10, while the axial force (the magnetic repulsion force between the stator steel plates) was about 0.1. By suppressing vibration of the stator in a radial direction thereof caused by the radial vibration force, noise and vibration can be effectively reduced.